Audio Engineering

540 Chapter 17

will be made on professional machines to switch between internal and external
references.

17.3.5 Error Correction and Concealment

As anyone familiar with analogue recording will know, magnetic tape is an imperfect
medium. It suffers from noise and dropouts, which in analogue recording are audible. In
a digital recording of binary data, a bit is either correct or wrong, with no intermediate
stage. Small amounts of noise are rejected, but inevitably, infrequent noise impulses
cause some individual bits to be in error. Dropouts cause a larger number of bits in one
place to be in error. An error of this kind is called a burst error. Whatever the medium and
whatever the nature of the mechanism responsible, data are either recovered correctly, or
suffer some combination of bit errors and burst errors. In compact disc, random errors
can be caused by imperfections in the moulding process, whereas burst errors are due to
contamination or scratching of the disc surface.

DAC

Address

compare

Motor

amplifier

Capstan

motor

Tape

Replay

head

Replay

circuits

Time compressed

off tape data

Data out at

reference frequency

Reference

Read clock

address

generator

Write

address

generator

Off tape clock Data

RAM

A

d

d

r

A

d

d

r

Figure 17.12 : In a recorder using time compression, the samples can be returned to a

continuous stream using RAM as a TBC. The long-term data rate has to be the same on the

input and output of the TBC or it will lose data. This is accomplished by comparing the read

and write addresses and using the difference to control the tape speed. In this way the tape

speed will automatically adjust to provide data as fast as the reference clock takes it from

the TBC.